There are lots of reasons why single women have sex with married men. I think that quite a common reason is because she feels she is reaching an age where she wants to be cherished and she looks around and there are no single men who are paying her any attention. But there might be a married man who is well put together, is used to familial responsibilities and is basically just nice to her whenever they encounter one another.
In this case, a single woman might then enter into an affair with a married man because she covets the life she perceives the wife is experiencing. Initially she might find the man attractive and over time, via regular interactions with the man at work or at social hobbies she might build up snippets of information into an imaginary lifestyle which she herself wants to live. She will then genuinely start to believe that this man will leave his wife for her and she will basically take the wife's place.
Just supposing the man is friendly, always happy, makes her laugh, does kind gestures, is seemingly nice to everyone else, drives a nice car, goes on nice holidays, runs his own business, etc. She's going to think 'lucky wife' and compare that to her own Saturday nights in with no partner watching the telly.
She won't give a thought to the wife being a person. The wife will become an obstacle to the object of her true desires. This may well be aided by the man barely - if at all - mentioning his wife in conversation, because he is physically attracted to the OW, fancies a different vagina, and reckons he might get away with it. He might build a subtle narrative to suggest to the OW that he needs saving (intimating that he does a lot of work around the house, works full-time, never mentions his wife's work - an indication to the OW that he needs saving from the awful shrew he is married to).
For me - and I would guess for a lot of people reading this forum - the idea of having an affair with a married man is preposterous. Disregarding any moral objections, which are valid in themselves, there is also the logical perspective to consider which is that the MM is in large part the sum of what his wife, his children, and his marriage have made him. So, if he is the attentive, kind, generous, funny man she covets, he probably is only this way because he has a supportive and loving base from which to be this way. If the OW were to be successful in pulling him from the clutches of his wife, the chances are she would end up with a very different character to the one she had created for herself in fairyland.
Yes I would feel sorry for the dumped OW in these circumstances. Not because she has been dumped, but because she was so lacking in her understanding of human relationships and in her ability to critically analyse the situation she was in.